Selective production of o-alkylphenols

ABSTRACT

In a method for the production of o-alkyl phenols by conversion of phenol with an alkanol at elevated temperature in the gas phase in the presence of a metal catalyst, the conversion takes place in at least two stages wherein the alkanol/phenol molar ratio in each reaction stage is set to a value of approximately ≦0.4; a clear increase in the selectivity for the o-alkyl phenol is obtained.

This application is a national stage entry under 35 U.S.C. §371 of PCTInternational Application No. PCT/EP02/12789, filed Nov. 15, 2002, whichclaims priority of German Patent Application No. 101 57 073.2, filedNov. 21, 2001.

The invention relates to a multistage method for the o-alkylation ofphenol by conversion of phenol with an alkanol at elevated temperaturein the gas phase in the presence of an acidic metal oxide catalyst.

Pure o-alkyl phenols are important compounds which are used in largequantities as starting material for organic chemical syntheses. Pureo-cresol (2-methyl phenol) is especially used for the production ofpesticides.

o-cresol can be obtained by methylation of phenol with methanol in thegas or liquid phase. As a result of the low reactivity of methanol, theconversion takes place at elevated temperature in the presence of acatalyst. Metal oxide catalysts such as aluminium oxide, silicondioxide/aluminium mixed oxide and magnesium oxide are used as catalysts.The choice of reaction temperature is made as a function of the catalystused in a range of 250 to 460° C. Thus, magnesium oxide exhibits a highselectivity for o-cresol in a temperature range of 420 to 460° C.whereas γ-aluminium oxide catalyses the methylation of phenol at atemperature of 200 to 400° C.

Also known is the production of o-cresol as a side product in thesynthesis of 2,6-dimethyl phenol and the subsequent isolation ofo-cresol by additional purification steps. Methods for the production ofo-cresol with further evidence are described in H. G. Franck, J. W.Stadelhofer, INDUSTRIELLE AROMATENCHEMIE, p. 170-177, SPRINGER VERLAG1987.

DE 27 56 461 A1 describes a generic method which is carried out at atemperature of 250 to 330° C. using alumina as the catalyst. An o-cresolyield of up to 26% is obtained with a methanol to phenol ratio of 0.5:1.The product contains an approximately 6% fraction of 2,6-dimethylphenol.

At high phenol conversions high fractions of 2,6-dimethyl phenol arealways obtained in addition to o-cresol. It is difficult to obtaino-cresol selectively by the methylation of phenol. Considerablequantities of m-cresol and p-cresol or higher alkylated products usuallyoccur as side products.

The high fraction of side products is common to all known methods forthe industrial synthesis of o-alkyl phenols.

The object of the invention is thus to provide a method for theproduction of o-alkyl phenol which is as selective as possible and canbe carried out on an industrial scale.

This object is achieved by o-alkylation of phenol with an alkanol atelevated temperature in the gas phase in the presence of a metal oxidecatalyst, in which the conversion is carried out in at least tworeaction stages and the alkanol/phenol molar ratio is maintained atapproximately ≦1 over the entire method.

The method according to the invention can be carried out, for example,in two to five stages. The method takes place especially preferably inthree stages. Each conversion stage can be carried out in a differentreactor. However, it is also possible to carry out several reactionstages in a single reactor. In this procedure several configurations ofthe active catalyst separated spatially from one another areaccommodated in the reactor. Between the catalyst configurations can bearranged zones with catalyst of lower activity or without catalyst.

The work forming the basis of this invention has shown that anisole isformed during the methylation of phenol and o-cresol is formed, on theone hand, by alkylation of phenol with anisole and, on the other hand,by intramolecular rearrangement of anisole to o-cresol. This finding isnew and contradicts the findings so far. The newly discovered reactionsequence is shown in FIG. 1.

It has surprisingly been found that multistage implementation of themethod under the afore-mentioned conditions yields a significantlyenhanced selectivity of the reaction for o-cresol and an associatedincrease in the yield of this compound. It is assumed that the highselectivity for o-alkylations is achieved by the low localconcentrations of methanol and therefore also of anisole, required inthis procedure. It is advantageous that the highly exothermic reactioncan be much better controlled by distribution over two, especially threereactors. The formation of so-called hot spots is thereby suppressed.

As a result of the multistage execution of the reaction, the alkanol tophenol ratio in each reactor or in each reaction stage can be setespecially low. The phenol conversion is thereby limited and aparticularly high selectivity for the o-alkyl phenol, for example,o-cresol can be achieved. The molar ratio of alkanol to phenol over theentire method is preferably set to 0.9, especially preferably to 0.6 ora value in between. Thus, the molar ratio of alkanol to phenol in athree-stage process is preferably 0.3 to 0.2 in each reaction stage.

Expressed alternatively, the phenol conversion can be set to a value ofapproximately 35 to 43%, for example, 38 to 42% in order to achieve thedesired high selectivity.

Suitable catalysts for implementing the method according to theinvention are acid metal oxides and their mixtures. Such metal oxidesare, for example, aluminium oxide, silicon dioxide/aluminium mixed oxideand magnesium oxide. Especially preferable is γ aluminium oxide. Thesurface area of the catalysts is preferably approximately 250 m²/g andmore, especially preferably 250 to 300 m²/g. These catalysts areproduced by known methods, for example, by ammoniacal hydrolysis ofaluminium nitrate and subsequent separation, drying and calcination ofthe precipitate obtained (J. Amer. Chem. Soc. 82 (1960) 2471).

The catalyst can be arranged in the usual form, for example, as a fixedbed, fluid bed or fluidized bed. The catalyst is preferably arranged ina fixed bed.

Alkanols to be used according to the invention are especially C₁₋₄alkanols, i.e., methanol, ethanol, n-propanol, isopropanol, n-butanoland isobutanol.

The method according to the invention can be implemented in atemperature range of 250 to 400° C. When γ aluminium oxide is used asthe catalyst, the temperature in the reactor is preferably 300 to 400°C., especially preferably 300 to 340° C., for example, 330° C.

An embodiment of the method according to the invention using methanol asthe alkylation agent is described subsequently as an example. The methodis carried out in three stages.

For this purpose phenol is fed into a mixer/vaporiser via a meteringdevice. Methanol is supplied to the same mixer/vaporiser via a meteringdevice. The mixture of initial compounds is fed into a tubular reactorheated to 330° C. The flow from the reactor contains the initialcompound phenol as well as the products and can be withdrawn at thebottom of the reactor. The flow is fed to another mixer/vaporiser whichis positioned before another tubular reactor. The methanol/phenol ratiorequired for the method according to the invention is adjusted again inthis mixer and the mixture obtained is fed into the second reactor. Theflow from the second reactor is either fed into a cooler or into a thirdmixer/vaporiser which is connected before a third reactor. Themethanol/phenol ratio required for the method according to the inventionis adjusted again in the third reactor and the mixture obtained is fedinto the third reactor.

The flow from the third reactor, if appropriate, from the secondreactor, is condensed in a cooler and the condensate is fed to a tank.

The processing of the raw alkylate can preferably take place bycontinuous rectification in a system consisting of three distillationcolumns connected one behind the other.

In the first column with, for example 20 to 35 practical plates whichare operated at normal pressure, the reaction water is separated at thehead of the column at a head temperature of 90 to 100° C. The watercontains small amounts of phenol, alkyl phenols and anisole since thesecompounds distill azeotropically with the water.

The sump of the first column is fed continuously into the second column.This column has approximately 100 practical plates and is also operatedat normal pressure. At the head of the column phenol with smallfractions of o-cresol is withdrawn at a head temperature of 180 to 185°C. This mass flow can be supplied to the alkylation stage again as rawmaterial.

The phenol-free sump of the column 2 continually enters the feed to thethird column with approximately 70 to 95 practical plates at 300 mbarhead pressure and approximately 145 to 155° C. head temperature. Pureo-cresol having a purity of >99.5% can be drawn off at the head of thecolumn.

The sump of the column which contains small fractions of o-cresol, canbe used as raw material to produce cresol/xylenol mixtures.

o-alkylated compounds to be obtained according to the invention arecresol and the ethyl, n-propyl, isopropyl, n-butyl and isobutylderivatives of phenol.

The following examples serve to explain the invention in greater detail.

EXAMPLE 1

It is known from the literature that anisole forms o-cresol byintramolecular rearrangement. For this reason it was taken as thestarting point that a high concentration of anisole should be built upto enhance the product yield. For this purpose pure anisole was fed intoa tubular reactor at a temperature of 330° C. at an LHSV (liquid hourlyspace velocity) of 1.25 h⁻¹ in a single-stage method. A γ aluminiumoxide with a surface area of approximately 250 m²/g is used as thecatalyst. The products obtained and their concentration in the productmixture are given in the following Table 1.

TABLE 1 Compound Concentration % Anisole 7.1 Phenol 32.2 o-cresol 28.62,6-xylenol 16.1 2,3,6-trimethyl phenol 4.0 Pentamethyl phenol 3.3Anisole conversion 92.9% o-cresol selectivity 30.8%

The results in the table show that anisole with a 92.9% conversion ishighly reactive under the selected conditions. The concentration ofphenol and higher alkylated phenols such as 2,6-xylenol and2,3,6-trimethyl phenol shows that only a portion of the anisole isrearranged into o-cresol. Most of the anisole reacts as an alkylationagent.

In order to confirm this supposition, an alkylation experiment wascarried out under the same conditions as those described previously, inwhich methanol was completely replaced by anisole. The results obtainedare given in Table 2.

TABLE 2 Alkylation agent Anisole Methanol Compound Concentration %Concentration % Anisole — — Phenol 76.8 72.3 o-cresol 18.8 20.52,6-xylenol 2.6 3.8 2,3,6-trimethyl — 0.4 phenol o-cresol 81.0% 74.0%selectivity

The values show that the behaviour of methanol is similar to that ofanisole. The use of anisole even shows a somewhat higher selectivitythan when methanol is used as the alkylation agent.

It can be postulated that the selectivity for o-cresol is approximatelythe same for both alkylation agents. The slightly higher o-cresolselectivity of anisole will be attributable to the fact that therearrangement of anisole to form o-cresol takes place at the same timeas the alkylation of phenol by anisole.

In order to study the fraction of o-cresol which is produced byintramolecular rearrangement, the same experiment was carried out using4-methyl anisole as the model substance. The concentration of theindividual products in the product mixture [%] and the 4-methyl anisoleconversion are given in Table 3.

TABLE 3 Compound Concentration % Phenol 57.7 o-cresol 12.8 p-cresol 18.62,6-xylenol 1.5 2,4-xylenol 6.4 4-methyl anisole conversion 100%

The results show that in addition to o-cresol, significantconcentrations of p-cresol and 2,4-xylenol were obtained. p-cresol isformed when 4-methyl anisole acts as an alkylation agent. 2,4-xylenol isthe product of the intramolecular rearrangement of 4-methyl anisole. Thecalculation shows that approximately 70% of the 4-methyl anisole acts asan alkylation agent and approximately 30% is rearranged to form2,4-xylenol. It is assumed that when anisole is used as the alkylationagent, the same ratios exist.

EXAMPLE 2

Phenol was pumped with methanol into the reactor at a reactortemperature of 330° C. with a methanol/phenol molar ratio of 0.2 and anLHSV of 3.75 h⁻¹. An γ aluminium oxide with a surface area ofapproximately 250 m²/g is used as the catalyst. In the following secondstage methanol was supplied in the molar ratio of 0.2 to the flow fromthe first stage and the alkylation continued. A similar procedure wasadopted to carry out the third alkylation stage.

For comparison, phenol with methanol was converted in a single-stagetubular reactor at a reaction temperature of 330° C. with amethanol/phenol molar ratio of 0.6 at an LHSV of 1.25 h⁻¹. The samecatalyst was used. In total the methanol/phenol ratio and the LHSV isthus the same as in the three-stage reaction. The product concentrationin the product mixture, the phenol conversion and the selectivity foro-cresol for both reactions are given in Table 4.

TABLE 4 Single-stage Three-stage Compound alkylation alkylation Anisole0.02 0.9 Phenol 53.7 58.6 o-cresol 27.9 29.4 m/p-cresol 1.5 0.82,6-xylenol 9.5 6.8 2,4/2,5-xylenol 2.0 0.9 2,3,6-trimethyl 1.7 0.9phenol Phenol conversion 46.3 41.4 o-cresol 60.3 71.1 selectivity

These results show that in the multistage method the selectivity foro-cresol is significantly higher than in the single-stage methodalthough the phenol conversion in the three-stage method is similar. Thereason for this is that the quantity of side products in thesingle-stage method is significantly higher than that in the three-stagemethod. Although the anisole content of 0.9% in this experiment is notyet optimal, an increase in the selectivity for o-cresol from 60.3% inthe single-stage method to 71.1% in the three-stage method is obtained.

1. A method for production of o-alkyl phenols by conversion of phenolwith an alkanol at elevated temperature in the gas phase in the presenceof a metal catalyst, characterized in that the conversion is carried outin at least two stages and the alkanol/phenol molar ratio in eachreaction stage is set to a value of approximately ≦0.4.
 2. The methodaccording to claim 1, characterized in that the conversion is carriedout in three stages.
 3. The method according to claim 1, characterizedin that the alkanol/phenol molar ratio in each reaction stage is set toa value of approximately 0.2 to 0.4.
 4. The method according to claim 3,characterized in that the alkanol/phenol molar ratio in each reactionstage is set to a value of approximately 0.3.
 5. The method according toclaim 1, characterized in that the phenol conversion during the reactionis set to 35 to 43% in each stage.
 6. The method according to claim 1,characterized in that methanol is used as alkanol.
 7. The methodaccording to claim 1, characterized in that an γ aluminum oxide having asurface area greater than 250 m²/g is used as the catalyst.
 8. Themethod according to claim 7, characterized in that the reaction iscarried out in a temperature range of 300 to 400° C.
 9. The methodaccording to claim 1, characterized in that the product mixture obtainedafter alkylation is separated by distillation to obtain the desiredproduct.